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Anal. Chem. 1986, 58, 1028-1032
differences in their hydrophobicity and redox behaviors). (1)
LITERATURE CITED Scoggins, E. A.; Maguire, K. P.;Norman, T. R.; Burrows, G. D. Clin.
Chem. (Winston-Salem, N . C . ) 1980, 2 6 , 5. (2) Suckow, R. R.; Cooper, T. B. J . Pharm. Sci. 1981, 70, 257. (3) "LCEC Application Note No. 40"; Bioanalytical Systems, Inc.: West Lafayette, IN. (4) Oelschlager, H. Bioelectrochem. Bioenerg . 1983, 10, 25. (5) Kalvoda, R. Anal. Chim. Acta 1982, 138, 11. (6) Wang, J. Am. Lab. (Fairfield, Conn.) 1985, 17(5),41. (7) Wang, J.; Freiha, E. A. Anal. Chem. 1984, 56, 849. (8) Wang, J.; Deshmukh, B. K.; Ronakdar, M J . Electroanal. Chem. 1985, 194, 339. (9) Jarbawi, T. 6.; Heinman, W. R.; Patrlarche, G. J. Anal. Chim. Acta 1981, 126, 57.
(IO) Wang, J.; Freiha, B. A., Deshmukh, E. K. Bioelectrochem. Bioenerg., in press. Jarbawi, T. B.; Heineman, W. R. Anal. Chlm. Acta, in press. Wang, J.; Freiha, E. A. Anal. Chim. Acta 1983, 148, 79. Wang, J.; Freiha, E. A. Anal. Chem. 1983, 55, 1285. Chaney, E. N.; Baldwin, R. P. Anal. Chem. 1982, 5 4 , 2556. Wang, J.; Freiha, 6.A. Bioelectrochem. Bioenerg. 1984, 12, 225. (16) Wang, J.; Deshmukh, E. K.; Bonakdar, M. Anal. Lett. Part8 1985, I8 (89),1087.
(11) (12) (13) (14) (15)
RECEIVED for review October 25, 1985. Accepted December 16,1985. This work was supported by the National Institutes of Health (Grants GM30913-02 and RR08136-12) and the American Heart Association.
Alternating Current Voltammetry of Dopamine and Ascorbic Acid at Carbon Paste and Stearic Acid Modified Carbon Paste Electrodes Mark B. Gelbert' and D. J. Curran* Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01002
The Oxidation of dopamlne in acldlc and pH 7 solutions has been examined at the carbon paste electrode by ac and dc cycllc voltammetry. The results of the ac experiments at pH 7 suggest that polymerlratlon of the amlnochrome formed during the dopamine oxldatlon may occur. The stearate modlfled carbon paste electrode was used to study the ac and dc cyclic voltammetry of dopamine and ascorbic acid at pH 7. Separation of the dopamine and ascorbate waves was complete wlth the latter shifted to potentials positive of the former. I n solutlons containing both dopamine and ascorblc acid, the dopamine oxldatlon peak Is enhanced conslderably more than would be posslble due to ring closure of the amlne side chain. The mechanism is clearly a catalyzed EC type where the dopamine-quinone formed by the electrochemlcal reaction Is reduced back to dopamlne by the ascorbate ion. Thus, ascorblc acid Is seen In its common role as an antioxidant. The catalyzed dopamine peak is sultable for quantltatlve purposes when the concentration of ascorblc acid is held constant.
Electroanalytical techniques have been used for in vitro and in vivo studies of catecholamines, which have produced considerable information about these compounds in the CNS (1). The ability to distinguish among the various catecholamines, their precursors and metabolites, and ascorbic acid in vivo has been a major goal in electroanalytical research for some time. Work has been reported where the electrochemical method or the type of electrode used has been varied to accomplish this. Lane et al. employed semidifferential electroanalysiswith carbon paste microelectrodes in the study of brain chemicals (2). Four distinguishable oxidation peaks were obtained from the caudate nucleus of the rat. The technique was latter applied by O'Neill and co-workers in an investigation of the rat stratium (3). Carbon fiber microelectrodes were first 'Present address: The Proctor & Gamble Co., Sharon Woods Technical Center, 11511 Reed Hartman Highway, Cincinnati, OH 45241.
reported by Gonon and co-workers ( 4 , s ) . They later electrochemically pretreated the electrodes at highly anodic potentials to achieve greater separation of the dopamine and ascorbic acid responses (6-10). Using differential normal pulse voltammetry and the treated electrodes, they achieved a peak separation of the two compounds of 190 mV (10). Ascorbic acid was the more easily oxidized and the sensitivity for dopamine was reported as 50 000 times that for ascorbic acid. Carbon electrodes of a different design were studied by Wightman and co-workers (11, 12). At their electrodes, a well-defined voltammetric response was obtained for dopamine and a draw-out response to ascorbic acid. Chemically modified electrodes have also been developed that separate the electrochemical responses of the catcholamines and ascorbic acid (13,14). A modified graphite paste electrode was prepared by mixing the Nujol paste with stearic acid (14). Electrostatic repulsion between the anionic carboxyl groups on the surface of the electrode and the ascorbate ion was considered to retard the rate of electron transfer and to be responsible for shifting the oxidation potential region of ascorbate to potentials more positive than that of dopamine. Alternating current voltammetry is a technique that is quite sensitive to the reversibility of the redox couple. Greater selectivity can be achieved relative to dc techniques by exploiting shifts in the peak potential and reduced sensitivity due to differing degrees of reversability between two given electroactive species. In the following work, the ac voltammetries of dopamine and ascorbic acid are explored at both the carbon paste electrode (CPE) and the stearic acid modified carbon paste electrode (MCPE). Cyclic voltammetry in the dc mode is used to confirm the results. Differences in the ac and dc results for dopamine at the CPE suggest the possibility of melanin formation. Results with the MCPE show complete separation of the dopamine and ascorbate waves and indicate a catalytic EC 'mechanism for the electrochemical oxidation of dopamine in the presence of ascorbate. EXPERIMENTAL SECTION Apparatus. The ac voltammetric potentiostat with digital phase sensitive detection was built in-house (15). A block diagram of the instrument is shown in Figure 1. The analog signal from
0003-2700/86/0358-1028$01.50/0 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
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RESULTS AND DISCUSSION Ac Voltammetry of Dopamine at the Carbon Paste Electrode. A cyclic voltammogram for the oxidation of dopamine in 0.1 M H2SO4on carbon paste and the corresponding dc voltammogram are shown in Figure 2. Both voltammograms exhibit nonreversible characteristics. The experimental ac peak current for the oxidation of lo4 M dopamine of 0.440 MAis nearly an order of magnitude smaller than that for the dc oxidation (3.94 PA). This illustrates the disadvantage of the ac approach relative to the dc technique in terms of analytical sensitivity when the redox system is not reversible. After eight voltammetric scans, the dc anodic peak current decreased by 3.2% and the ac peak current decreased by 3.6%. This indicates little loss in sensitivity a t the carbon paste electrode with repeated scans and suggests that there is no
I
1
800
600 E (mV)vs SCE
Flgure 1. Block diagram of the ac potentiostat with digital lock-in amplification.
the tuned amplifier is converted to a digital signal with a V/F converter so the output frequency is proportional to the amplitude of the input sine wave. The pulses are counted by a set of up/down counters, which are controlled by timing circuitry triggered by a reference square wave derived from the internal oscillator. The net count is converted to voltage by a DAC, which is connected to the X-Y recorder. A more detailed description of the instrument is available (15). Electrodes, The carbon paste electrode was constructed with a Teflon plug serving as a holder for the paste, a glass barrel to which the plug is attached, and a copper rod for electrical contact. The diameter of the carbon paste well was 0.25 in. The paste consisted of Ultra Carbon USP graphite powder and Nujol in the ratio of 1.65 to 1 by weight. The paste was prepared by taking the graphite/Nujol mixture and wetting it with 10 mL of spectroscopic grade toluene and stirring well. The toluene was blown off by a stream of purified nitrogen gas and the paste was then placed under vacuum in a desiccator for 1 2 h. The stearatemodified carbon paste was made by mixing 100 mg of stearic acid with 1.5g of graphite and 1mL of Nujol. The mixture was wetted with 10 mL of toluene and dried as described. Chemicals. Dopamine hydrochloride and stearic acid (99%) were obtained from Sigma Chemical Co.; Catechol and L-ascorbic acid (99.5%)were obtained from Fisher Scientific Co. All were used as received. Potassium phosphate, monobasic sodium hydroxide solution from Fisher was used as the pH 7 electrolyte. The other electrolyte used was 0.1 M HzSO4 made from reagent grade HzS04 and distilled deionized water obtained from a Barnstead water purification cartridge system. Procedures. For all ac voltammetric experiments, f = 10 Hz, AE = 5 mV, and u = 5 mV/s. Dc scans were done at the same sweep rate, 5 mV/s. The concentration of dopamine, ascorbic acid, and catechol was M unless otherwise noted. Solutions of variable ascorbic acid concentration and constant dopamine concentration were prepared by pipetting incremental amounts of a solution of dopamine and ascorbic acid into 50 mL of the supporting electrolyte containing dopamine.
I
I
400
A
i\
T
I
0.13 UA
1
I 200
I
I
I
I
600
400
I
1 800
E (mV) vs SCE Figure 2. Dc (A) and ac (B) voltammograms for the oxidation of
dopamine on carbon paste in sulfuric acid. Table I. Dc and Ac Peak Current as a Function of Repetitive Scans and Renewed Surfaces for the Oxidation of Dopamine in HzSOl Dc
scan no.
i,(dc), .uA
surface no.
i,(dc)," WA
1 2 3 4 5 6 7 8
3.74 3.52 3.64 3.64 3.64 3.66 3.64 3.62
1 2 3 4 5 6 7
4.00 3.94 3.76 4.14 3.86 3.80 4.08
I,(ac), pA
surface no.
I,,(ac),b pA
0.437 0.431 0.437 0.437 0.434 0.424 0.424 0.421
1 2 3 4 5 6 7
0.440 0.460 0.440 0.453 0.438 0.447 0.405
Ac
scan no.
"i,(av) = 3.94 1 0.13 pA, 1 3 . 4 % . bZp(av)= 0.440 & 0.016 FA, f3.7%.
electrode fouling due to the oxidative process. The reproducibility of the peak current a t new carbon paste surfaces as measured by the relative standard deviation (RSD) was f3.4% for the dc experiment and *3.7% for the ac experiments (Table I). The ac and dc voltammograms for the oxidation of dopamine in pH 7 phosphate buffer are shown in Figure 3. These current-voltage curves are very different from those shown
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 6 , MAY 1986
A
B
A
T
A
08UA
1
T 0.8 UA
1
3
I -100
-
4
7
0
100
200
0
300 400 E (mV)vsSCE
100
200
300
E (mV)vs SCE
Flgure 3. Dc (A) and ac (B) voltammograms for the oxidation of dopamlne on carbon paste at pH 7.
Table 11. Dc and Ac Peak Current as a Function of Repetitive Scans and Renewed Surfaces for the Oxidation of Dopamine at pH 7 scan no.
i,(dc), PA
1 2
5.16 4.84
3
4.22
4
4.00 3.94 3.76 3.76 3.72
5 6 7 8
surface no.
i,(dc),” rA
1
5.06
2 3
5.02 5.00
5.16 5.08 5.26 5.06
Ac
scan no.
Ip(ac),P A
surface no.
I,(ac),* r A
1 2
0.958 0.871 0.765 0.668 0.604 0.543 0.498 0.478
1 2
1.03 0.993 0.948 1.23 1.34 1.23 0.961
Oi,(av) = 5.09 i 0.08 FA, i1.6%. bIp(av)= 1.11 i 0.146 wA, i13.3%. in Figure 2. The dc anodic peak current is about 29% larger than that for the same concentration of dopamine in the acidic solution (5.09 pA VI. 3.94 PA). Similarly, the ac peak current has increased by 152%. The explanation for this is well-known from the work of Adams and co-workers, who described the ECC nature of the oxidation of catecholamines at this pH (16). Upon loss of a proton, the dopamine-quinone undergoes cyclization of the amine side chain to form the corresponding aminochrome, which can react chemically with the oxidized dopamine to produce the starting material, dopamine. Thus, the apparent number of electrons transferred in the overall reaction is between 2 and 4. On subsequent scans with the same electrode surface, the peak currents for the dc and ac voltammograms decreased considerably: 30% for the dc case and 50% for the ac voltammogram (Table 11). The reproducibility of the peak current with renewed carbon paste surfaces was i l . 6 % RSD for the dc experiments and f 1 3 % for the ac peaks. These
i 0
i 100
i 200
E (mV) vsSCE
300
~ 400
i
o 0
i 100
i 200
300
i 400
E ( m V ) v s SCE
Flgure 4. Dc (A) and ac (B) voltammograms for the oxidation of catechol on carbon paste at pH 7.
results are also much different from the results obtained in acid solution. The poorer precision of the ac experiments was of particular interest and an explanation was sought. Lane and Hubbard reported a poisoning effect on platinum electrodes from the oxidation of dopamine at physiological pH (17). Their conclusion, based on previous chemical evidence reported in the literature, was that the product of the ring closure, the aminochrome, can polymerize to melanin-like compounds on the surface of the electrode and, thus, inhibit the electron transfer reaction. The literature suggests that melanin is formed by a free radical polymerization (18). This conclusion is based on finding trapped free radicals in natural and synthetic melanins. The random structural arrangement of the melanin also indicates a free radical mechanism. Clearly, a free radical semiquinone is a possible intermediate in the oxidation of a hydroquinone. Such free radical intermediates could bond to the growing melanin, producing a rather random structure for the polymer. For the oxidation of dopamine a t pH 7, the dopamine-quinone product can be electrochemicallyreduced back to dopamine in the ac diffusion layer or it can undergo the ring closure reaction. Further, if melanin formation does occur and a free radical polymerization involving the semiquinone intermeidate is involved, the concentration of dopamine-quinone present at the surface of the electrode could vary from experiment to experiment although each experiment is carried out under the same conditions. This offers a possible explanation of why the ac experiment is reproducible in the acidic solution but not at pH 7. Further, the ac measurement is more sensitive than the dc measurement to the kinetics of following chemical reactions (19). Further evidence to support this hypothesis was obtained by examining the oxidation of catechol, which does not have the amine side chain and cannot undergo ring closure after electrochemical oxidation. The ac and dc voltammograms at pH 7 are shown in Figure 4. The decrease in anodic peak current after eight repeated scans on the same electrode surface was 7.8% for the dc case and 5.8% for the ac case (Table 111). The RSD of the peak currents on renewed carbon paste surfaces was &2.0%for the dc work and *2.2% for the ac voltammograms (Table 111). These results show that the poor reproducibility of the ac peak current for the oxidation of dopamine at pH 7 can be attributed to the presence of the amine side chain and therefore involves the chemical reactions that can take place after the dopoamine-quinone is formed. Electrochemistry of Dopamine a n d Ascorbate Ion at the Stearate Modified Carbon P a s t e Electrode. The stearate modified carbon paste electrode was first reported by Blaha and Lane (14). Using chronoamperometry, they found the currents for dopamine and ascorbic acid at pH 7.4 to be in the ratio 1000/1 a t 0.25 V vs. Ag/AgC1 and 111/1at
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
1031
T 8 uA
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1
il
1
I
I
0
I
I
I
I
I
400
200
I
I
800
600
I
T
E (mV) vs SCE
0.26uA
Figure 5. Dc voltammograms for the oxidation of dopamine (A) and ascorbic acid (B) on a stearate modified carbon paste electrode at pH
1
7.
n
I 100
I
I
I
I
I
1
0
100
200
300
400
500
E (mV)vs SCE
T
Flgure 7. Dc (A) and ac (B) voltammograms for the oxidation of
6 4 nb
dopamlne with ascorbic acid present at 10 times the dopamine concentration, pH 7.
1
T 13 n A
1 I
I
0
I
I
200
I
I
I
400
I
600
I
I
t
800
E (mV) vs SCE
T 0 YA
Flgure 6 , Ac voltammograms for the oxidation of dopamine (A) and ascorbic acid (B) on a stearate modified carbon paste electrode at pH 7.
0.45 V. No current-voltage curves were presented and the mechaism at this electrode for the electrochemical oxidation of dopamine in the presence of ascorbic acid was not discussed. The MCPE is extremely successful in separating the oxidation waves of dopamine and ascorbate at pH 7 as shown in Figures 5 and 6 for the dc and ac cyclic experiments, respectively. It is important to note that these voltammograms were obtained by using separate solutions of dopamine and ascorbic acid. The ascorbate wave is shifted to potentials positive of the dopamine wave and its ac peak current is almost an order of magnitude smaller (25.5 nA vs. 246 nA) than that of dopamine although the concentration of each compound is the same. The latter can be explained on the basis of the EC nature of the mechanism for the oxidation of ascorbate (10,20). At the peak potential of the dopamine ac wave, there is virtually no response to ascorbate. However, the ac peak current for dopamine is smaller than that observed at the ordinary carbon paste electrode. When solutions containing both dopamine and ascorbic acid at pH 7 were examined, the shape of the voltammogram was found to change dramatically (Figure 7 ) . The magnitude of the peak current for dopamine was greatly enhanced. In 1976, Adams and co-workers reported evidence that clearly showed ascorbic acid would reduce dopamine-quinone back to dopamine (21). However, the work was done at a CPE and it was not possible to observe enhancement of the dopamine peak. This reaction prevents the formation of aminochromes and retards the nucleophilic substitution of catecholamines, which could produce powerful neurotoxins such as 6-hydroxydopamine (21). Other evidence exists in the literature for the strongly reducing nature of ascorbic acid in biological matrices
02
04 E IV)
VS
06 SCE
08
10
12
Figure 8. Dc voltammograms for a mixture of M dopamine and IOw3M ascorbic acid (A) and of M ascorbic acid alone (B).
0.13 1 T YA
t 02
0.4
06
08
10
12
E ( V I VS SCE
Flgure 9. Ac voltammograms for the solutions of Figure 8.
(22-27). At pH 7, it is calculated to have a formal potential of approximately 0.059 V (22). Thus, the mechanism at the MCPE would be predicted to be of the catalytic EC type. The voltammograms in Figures 8 and 9 bear this out. The dc cyclic
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
1032
Table 111. Dc and Ac Peak Current as a Function of Repetitive Scans and Renewed Surfaces for the Oxidation of Catechol at pH 7
1.o
Dc scan no.
i,(dc), FA
1 2
4.14 4.02 4.00 4.00 4.02 3.96 3.96 3.96
3 4 5
6 7
8
surface no.
i,(dc)," FA 4.08 4.28 4.30 4.16 4.20 4.26 4.34
Ac scan no.
I,(ac), W A
surface no.
I,(ac),bFA
1 2
0.284 0.278 0.273 0.270 0.268 0.268 0.265 0.265
1 2
0.280 0.270 0.286 0.267 0.273 0.273 0.286
3 4 5 6 7 8
3 4 5
6 7
'iJa57) = 4.23 i= 0.08 FA, f 1.97%. bIp(av)= 0.276 i= 0.006 FA, rt2.2%.
Q2 i
2
4
6 0 conc. DA COnc AH2
1
0
Flgure 10. Plot of the ac peak current for the oxidation of dopamine in the presence of ascorbic acid vs. the concentration ratio of dopamine to ascorbic acid.
of the ac peak current sensitivity (amperes/molar) for dopamine to that for ascorbic acid, together with the peak potential separation achieved for these two compounds, makes the MCPE worthy of further application to studies of the electrochemistry of catecholamines in the presence of ascorbic acid. The work also suggests that one possible role for ascorbic acid in the central nervous system is to prevent the formation of melanins. Registry No. Dopamine, 51-61-6; ascorbic acid, 50-81-7; carbon, 7440-44-0; stearic acid, 57-11-4; catechol, 120-80-9;graphite, 7782-42-5. LITERATURE CITED
shows no current for the reduction of the dopamine-quinone on the cathodic scan and very little current for the oxidation of ascorbate. The ac cyclic shows more clearly some oxidation current for ascorbate and some reduction current for dopamine-quinone. Again, this is because of the shorter time frame of the ac experiment. By shifting the ascorbate wave to potentials positive of the dopamine wave, the MCPE electrode is able to demonstrate clearly the nature of the chemistry that is occurring. Ascorbate acts as an antioxidant, quite possibly to prevent melanin formation in biological situations. The peak current enhancement depends on the concentration of ascorbate and is too large to be accounted for on the basis of an ECC mechanism. Peak currents were measured for solutions containing concentration ratios of dopamine to ascorbic acid ranging from 10/1 to 1/10 and the results are plotted in Figure 10 for the ac experiments. Similar results were found for the dc experiments. At an ascorbic acid concentration 10 time larger than the dopamine concentration, the dc peak current was increased by $fold and the ac peak current by over 4-fold. The catalyzed peak current was somewhat less reproducible than uncatalyzed peak currents observed earlier in the work. The average RSD over the range of concentration ratios studied was a little less than f 7 % . A working curve for the determination of dopamine over the M in the presence concentration range of about (1to 20) X of 5 times the concentration of ascorbic acid was linear with a slope and correlation coefficient of 7.32 X AIM and 0.9965, respectively. This suggests that very dilute solutions of dopamine might be determined by adding ascorbic acid to the solution to enhance the dopamine response. It could be done by postcolumn mixing in HPLC analyses of catecholamines.
(1) Marsden, C. A,; Brazell, M. P.; Maidment, N. T. I n "Measurement of Neurotransmitter Release I n Vivo"; Marsden, C. A,, Ed.; Wiley: New York, 1984. (2) Lane, R. F.; Hubbard, A. T.; Blaha, C. D. J . Necfroanal. Chem. 1979, 9 5 , 117. (3) O'Neill, R . D.; Grunewald, R. A.; Fiilenz, M.; Albery, W. J. Neuroscience 1982, 7 , 145. (4) Gonon, F.; Cespuglio, R.; Ponchon, J.-L.; Buda, M.; Jouvet, M.; Adams, R. N.; Pujol, J.-F. C.R. Hebd. Seances Acad. Sci., Ser. D 1978, 286, 1203. (5) Ponchon, J.-L.; Cespuglio, R.; Gonon. F.; Pujol, J.-L. Anal. Chem. 1979, 5 1 , 1483. (6) Gonon, F.; Buda, M.; Cespuglio, R.; Jouvet, M.; Pujol, J.-F. Nature (London) 1080, 286, 902. (7) Gonon, F.; Buda, M.; Cespuglio, R.; Jouvet, M.; Pujol, J.-F. Brain Res. 1981, 223, 69. ( 8 ) Gonon, F. G.; Fombarlet, C. M.; Buda, M. J.; Pujol, J.-F. Anal. Chem. 1981, 5 3 , 1386. (9) Gonon, F.; Buda, M.; DeSimoni, G.; Pujol, J.-F. Brain Res. 1983, 273, 207. 10) Gonon, F. G.; Navarre, F.; Buda, M. J. Anal. Chem. 1984, 56, 573. 11) Wightman, R. M. Anal. Chem. 1981, 5 3 , 1125A. 12) Ewing, A. G.; Wightman, R. M.; Dayton, M. A. Brian Res. 1982, 249, 361. 13) Ravichardran, K.; Baldwin, R. P. Anal. Chem. 1983, 55, 1586. 14) Blaha, C. D.; Lane, R. F. Brian Res. Bull. 1983, 10, 861. 15) Kingsley, E. D. Ph. D. Dissertation, University of Massachusetts, Amherst, MA, 1982. Hawiey, M. D.; Tatawawadi, S.V.; Piekarski, S.;Adams, R. N. J . Am. Chem. SOC. 1967, 89, 447. Lane, R. F.; Hubbard, A. T. Anal. Chem. 1976, 4 8 , 1287. Bloise, M. S., Jr. I n "Solid State Biophysics"; Wyard, S. J., Ed., McGraw-Hill: New York, 1969; pp 243-262. Bullock, K. R.; Smith, D. E. Anal. Chem. 1974, 46, 1567. Ruiz, J. J.; Aidaz, A,; Dominguez, M. Can. J . Chem. 1977, 55, 2799. Tse, D. C. S.; McCreery, R. L.; Adams, R . N. J . Med. Chem. 1978, 19, 37. Erdev, L.; Svehia, G. "Ascorbinometric Titrations"; Akademiai Kiado: Budapest, 1973; pp 9-20. (23) Stein, L.; Wise, C. D. Science 1971, 171, 1032. (24) Petrack, B.; Sheppy, F.; Fetzer, V. J . Bioi. Chem. 1968, 243, 743. (25) Lerner. P.; Hartman, P.; Ames, M. M.; Lovenberg, W. Arch. Biochem. Biophys. 1977. 182, 164. (26) Navon, A. J . Insect Physioi. 1978, 2 4 , 39. (27) Cottrell, C. B. Adv. Insect Physioi. 1964, 2 , 175.
CONCLUSIONS This work demonstrates that ac and dc cyclic voltammetry complement each other nicely in qualitative investigations of electrode reactions. The ac method has the advantage of working on a shorter time scale without distortion of the current-voltage curve by charging current. The 100/ 1 ratio
RECEIVED for review July 1,1985. Resubmitted December 2, 1985. Accepted December 2,1985. M.B.G. wishes to express his thanks for support in the form of a Stauffer Fellowship awarded by the chemistry department. Work was also supported by a grant from the Graduate School, University of Massachusetts.
